The present invention relates to switching circuits and, more particularly, to a circuit that drives a switching device such as the gate of either an integrated circuit or discrete power MOSFET for reducing the turn off delay of the MOSFET while limiting its gate current to maintain a controlled rise (dv/dt) of the drain voltage.
A myriad of circuit applications can be found in the prior art which utilize a gate driven MOSFET that is switched between conducting and non-conducting operating states. The MOSFET, which may be either a discrete device or an integrated device typically has its drain-source electrodes coupled in series with a load element between a power supply. The MOSFET has its gate driven by a controlled current switching circuit such that the load element is connected to the power supply through the low on-resistance of the MOSFET when the latter is turned on as is well known.
Most, if not all, such circuit applications suffer from a time delay resulting as the MOSFET is turned off to disconnect the load element from the power supply. Turn-off delay is a problem for controlled current gate drive circuits because the gate must be discharged from a relatively high V.sub.GS (typically 10 volts) to the threshold V.sub.GS (typically 4.0 volts) before the drain voltage begins to rise as the device is turned off. If a constant current is used to control the dv/dt voltage rise at the drain of the MOSFET, then the time required to slew the gate from 10 volts to 4 volts is excessive. Previous attempts to reduce the delay have used rapid discharge circuits that discharge the gate to a predetermined voltage. These discharge circuits must initially conduct relatively high currents but stop conducting when the gate reaches the threshold voltage. However, the threshold voltage of a given MOSFET varies in production of such applications with the load element, temperature and FET V.sub.T, hence, it is impossible to design a common discharge circuit of the aforementioned type that works well under all conditions.
Thus, a need exists for providing a circuit and method for overcoming the problems of prior art circuits and to reduce the turn-off delay time required in circuit applications utilizing MOSFET switched devices while controlling the dv/dt rise at the drain thereof.